The proposed research involves characterization of large protein complexes such as vault proteins using mass spectrometry (MS) and proteomics to elucidate structure and biological function. Vaults are linked to multidrug resistance (MDR), the major cause for chemotherapy failure in cancer treatment. Although the function of vaults remains unknown, several functions are proposed such as a cellular transporter or in signaling pathways for cell survival or growth. Identification of vault interacting proteins and vault contents using MS, immunoprecipitation, chemical cross-linking, and/or tandem affinity purification (TAP) tagging of the major vault protein (MVP) will lead toward the elucidation of its interacting components, structural assembly, and biological function. This will also confirm the role of vaults in MDR if drugs are found in vault derived MDR established cell lines. Since MVP is a novel substrate of a protein tyrosine phosphatase (SHP-2), the phoshorylation sites of MVP will be identified by 2D-PAGE, and stained for phosphoproteins, trypsin digested, enriched by affinity chromatography or an automated method by a titanium oxide column, and analyzed by HPLC-ESI-MS/MS, only requiring femtomolar levels of phosphopeptides.